1. Field of the Invention
The present invention relates to a signal processing apparatus for processing video signals for the purpose of .gamma.-correction or the like.
2. Description of the Prior Art
Reproduction of a color television image is conducted by controlling electron beams from three electron guns of a color cathode ray tube, thus causing light emissions from red, green and blue phosphors. If the grid signal voltage supplied to the grid of the image tube and the amount of light emission are in a linear relation so that the latter is in proportion to the former, the signal voltages directly correspond to the stimulus values of red, green and blue light. In fact, however, the amount of light emission lacks such linearity, and is proportional to about 2.2 powers of the signal voltage.
For this reason, the signal voltages obtained in a television camera in proportion to the stimulus values of an object will, if supplied directly to the cathode ray tube of such non-linear characteristic, reproduce a color image distorted not only in luminance but in hue and saturation. In a color television system, therefore, it is necessary to supply the signal to the cathode ray tube through a non-linear circuit with an output-to-input characteristic equal to the power of 1/2.2, whereby the overall characteristic of the system including that of the cathode ray tube becomes linear.
The relation between the light emission L and the grid signal voltage E in a color cathode ray tube can be approximated by the following equation: EQU L=kE.sup..gamma.
wherein k is a constant. .gamma. is the slope of L plotted against E with both in logarithmic scale, which is called the .gamma.-characteristic of the cathode ray tube. .gamma. is usually 2.2 for a color cathode ray tube. Such correction for the .gamma.-characteristic is best conducted in the television receiver, but is generally effected on the output signal of the television camera in order not to complicate the circuits of the television receiver. The circuit for the above-mentioned non-linear correction is called the .gamma.-correction circuit.
However, the conventional .gamma.-correction circuit is not only complicated in structure but also is a bar to designing a compact and sample image pickup unit since such correction circuits are needed both for the luminance signal and for the color circuit is needed for each of the luminance signal and color difference signal.
Particularly in the case of obtaining video signals from a semi-conductor image pickup device, such .gamma.-correction will intolerably reduce the dynamic range of the signals since the charges generated in such image pickup device only have a limited dynamic range.
Also in combination with the digitization of video signals recently developed, the .gamma.-correction, if applied after analog-to-digital conversion, will require not only a complicated correction circuit but also a complicated synchronization control.